A monopulse tracking system is a relatively well known direction finding or antenna pointing system utilizing a special monopulse antenna having at least two elements situated to receive a signal from a remote source, the direction of which relative to the antenna boresight axis it is desired to determine. The elements of the antenna are situated so that the signal is received in both elements in phase if the antenna is pointing directly at the remote source and the signal is received out of phase in varying amounts in accordance with the amount of error in the direction the antenna is pointing. In general, the two elements of the monopulse antenna provide a single axis of direction and if a second, quadrature axis is desired additional elements must be utilized in the antenna. Typical monopulse antennas are illustrated in U.S. Pat. No. 3,990,078, issued Nov. 2, 1976, entitled Image Element Antenna Array for A Monopulse Tracking System for A Missile, and U.S. Pat. No. 4,309,706, issued Jan. 5, 1982, entitled "Wide Band Direction Finding System".
A major difficulty experienced in prior art monopulse tracking systems is encountered in the presence of externally generated noise. Externally generated noise creates interference in both the sum and the difference channels and generally remain after down conversion, phase detection and filtering. Since the externally generated noise components in the sum and difference channels exhibit common features, there is a correlation of the noise in the phase detector which produces bias errors (DC offset) that cannot be removed by the filtering. The bias errors are developed, even if the link is protected by spread spectrum means because the externally generated noise is spread by the receiver despreading function again producing noise in the sum and difference channels which will correlate in the phase detector because of the use of the common despreading function.